Capturing Nucleation at 4D Atomic Resolution*

Nucleation is a ubiquitous phenomenon in many physical and biological processes. However, it is a challenging process to study nucleation due to the lack of experimental tools to directly measure the 3D atomic structure of nuclei. Here, we further advance atomic electron tomography to study the early stage nucleation of FePt nanoparticle system at 4D atomic resolution [1]. We reveal that early stage nuclei are irregularly shaped, each has a core of a maximum order parameter, and an order parameter gradient points from the core to the boundary of the nucleus. We also capture the 3D atomic structure and dynamics of the same nuclei undergoing growth, fluctuation, dissolution, merging and/or division. We find that nucleation dynamics is regulated by the distribution of the order parameter and its gradient. These experimental results differ from classical nucleation theory (CNT), and we propose an order parameter gradient nucleation model which is thermodynamically more favorable than CNT. We further corroborate this model using molecular dynamics simulations of the liquid-solid phase transition of Pt.
[1] J. Zhou et al., arXiv:1807.10709 (2018).